The invention includes novel chemical compounds having specific binding in a biological system and capable of being used for positron emission tomography (PET) and single photon emission (SPECT) imaging methods.
The ability of analog compounds to bind to localized ligands within the body makes it possible to utilize such compounds for in situ imaging of the ligands by PET, SPECT and similar imaging methods. In principle, nothing need be known about the nature of the ligand, as long as binding occurs, and such binding is specific for a class of cells, organs, tissues or receptors of interest. PET imaging is accomplished with the aid of tracer compounds labeled with a positron-emitting isotope (Goodman, M. M. Clinical Positron Emission Tomography, Mosby Yearbook, 1992, K. F. Hubner et al., Chapter 14). For most biological materials, suitable isotopes are few. The carbon isotope, [.sup.11 C], has been used for PET, but its short half-life of 20.5 minutes limits its usefulness to compounds that can be synthesized and purified quickly, and to facilities that are proximate to a cyclotron where the precursor [.sup.11 C] starting material is generated. Other isotopes have even shorter half-lives. [.sup.13 N] has a half-life of 10 minutes and [.sup.15 O] has an even shorter half-life of 2 minutes. The emissions of both are more energetic than those of [.sup.11 C]. Nevertheless, PET studies have been carried out with these isotopes (Hubner, K. F., in Clinical Positron Emission Tomography, Mosby Year Book, 1992, K. F. Hubner, et al., Chapter 2). A more useful isotope, [.sup.18 F], has a half-life of 110 minutes. This allows sufficient time for incorporation into a radio-labeled tracer, for purification and for administration into a human or animal subject. In addition, facilities more remote from a cyclotron, up to about a 200 mile radius, can make use of [.sup.18 F] labeled compounds. Disadvantages of [.sup.18 F] are the relative scarcity of fluorinated analogs that have functional equivalence to naturally-occurring biological materials, and the difficulty of designing methods of synthesis that efficiently utilize the starting material generated in the cyclotron. Such starting material can be either fluoride ion or fluorine gas. In the latter case only one fluorine atom of the bimolecular gas is actually a radionuclide, so the gas is designated .sup.18 F-F. Reactions using .sup.18 F-F as starting material therefore yield products having only one half the radionuclide abundance of reactions utilizing K.sup.18 F as starting material. On the other hand, [.sup.18 F] can be prepared in curie quantities as fluoride ion for incorporation into a radiopharmaceutical compound in high specific activity, theoretically 1.7 Ci/nmol using carrier-free nucleophilic substitution reactions. The energy emission of [.sup.18 F] is 0.635 MeV, resulting in a relatively short, 2.4 mm average positron range in tissue, permitting high resolution PET images.
SPECT imaging employs isotope tracers that emit high energy photons (.gamma.-emitters). The range of useful isotopes is greater than for PET, but SPECT provides lower three-dimensional resolution. Nevertheless, SPECT is widely used to obtain clinically significant information about analog binding, localization and clearance rates. A useful isotope for SPECT imaging is [.sup.123 I], a .gamma.-emitter with a 13.3 hour half life. Compounds labeled with [.sup.123 I] can be shipped up to about 1000 miles from the manufacturing site, or the isotope itself can be transported for on-site synthesis. Eighty-five percent of the isotope's emissions are 159 KeV photons, which is readily measured by SPECT instrumentation currently in use.
Use of [.sup.18 F] labeled compounds in PET has been limited to a few analog compounds. Most notably, [.sup.18 F]-fluorodeoxyglucose has been widely used in studies of glucose metabolism and localization of glucose uptake associated with brain activity. [.sup.18 F]-L-fluorodopa and other dopamine receptor analogs have also been used in mapping dopamine receptor distribution.
Other halogen isotopes can serve for PET or SPECT imaging, or for conventional tracer labeling. These include .sup.75 Br, .sup.76 Br, .sup.77 Br and .sup.82 Br which have usable half-lives and emission characteristics. In general, the chemical means exist to substitute any halogen moiety for the described isotopes. Therefore, the biochemical or physiological activities of any halogenated homolog of the described compounds are now available for use by those skilled in the art, including stable isotope halogen homologs. Astatine can be substituted for other halogen isotopes; [.sup.210 At] emits alpha particles with a half-life of 8.3 h. At-substituted compounds are therefore useful for tumor therapy, where binding is sufficiently tumor-specific.
Serotonin transporters are proteins that reside on the membrane of the nerve terminals of the presynaptic serotoninergic neurons. The serotonin transporter binds serotonin released during synaptic transmission by serotoninergic neurons, removing serotonin from the synapse. The serotonin transporter therefore serves to regulate the level of neurotransmitter in serotoninergic nerve transmission. Serotonin transporter is known to be involved in the pathophysiology of major depression and is the site of action of the majority of antidepressants, [See Murphy, D. L. et al. (1986) J. Clin. Psychiatr. 47:(suppl)9-15]. Supporting evidence for serotonin transporter function has been indirect, based on studies of post mortem tissue and of animal and peripheral cell models of transporter cell function and pharmacology. Direct evidence from emission tomography is needed to further define the functional status and pharmacology of serotonin transporter and antidepressant medication in the living human brain. A valuable tool for such studies would be the development of imaging agents labeled with positron emitters as probes to study the in vivo function of this neuroregulatory site by PET, or by SPECT using .gamma.-emitting probes.
Currently, there does not exist a single radiopharmaceutical that can be labeled with either fluorine-18 and iodine-123 amenable for regional distribution that is efficacious in differentiating major depression from other psychiatric disorders. Citalopram [Hume et al. (1991) Nucl. Med. Biol. 18:339-351], paroxetine [Suehiro et al. (1991) Nucl. Med. Biol. 18:791-796], fluoxetine [Kilbourn et al. (1989) J. Label. Cmpd. Radiopharm. 26:412-414], and nitroquipazine [Mathis et al. (1993) J. Nucl. Med. 34:7P-8P], potent serotonin transporter ligands, have been radiolabeled with carbon-11 and fluorine-18 as potential radiotracers for localizing and quantifying serotonin transporter sites in the brain using PET. Unfortunately, the in vivo affinity and selectivity for the serotonin transporter of these radiolabeled ligands did not reflect their in vitro potencies as reflected in poor quality images of brain regions rich in serotoninergic neurons. Recently, a series of trans-1,2,3,5,6,10b-hexahydro-pyrrolo(2,1-a]isoquinoline derivatives, have been found to be potent inhibitors with low and subnanomolar affinity for the serotonin transporter [Maryanoff et al. (1987) J. Med. Chem. 30:1433-1454]. The most potent inhibitor of the series, trans-1,2,3,5,6,10b-hexahydro-6-[4-(methylthio)phenyl]pyrrolo[2,1-a]isoqui noline (McN-5652Z) (Ki=0.68 nM), has been labeled with carbon-11 for use as a PET radioligand for mapping serotonin transporter sites [Suehiro et al. (1992) J. Label Cmpd. Radiopharm. 31:841-848]. Carbon-11 McN-5652Z showed the greatest accumulation in brain regions rich in serotoninergic neurons with greater cortex to cerebellum ratios (4.3 to 1) than previously tested PET serotonin transporter ligands [Suehiro et al. (1993) J. Nucl. Med. 34:120-127. However, the very short 20 minute half-life of carbon-11 is not ideal for longitudinal selective regional uptake of the radioligand and the presence of radiolabeled metabolites that is crucial in binding site imaging and tracer kinetic modeling. Thus, there exists a need for a probe with a longer half-life that demonstrates sub to low nanomolar affinity, high selectivity, and a low dissociation rate from the serotonin transporter binding site. Because the serotonin transporter plays a pivotal role in serotonin neurotransmission, the development of radiopharmaceuticals radiolabeled with gamma or positron emitting isotopes which exhibit pronounced brain uptake, very high selectivity and affinity for the transporter, and low nonspecific binding would be excellent for the measurement of the density of presynaptic serotonin transporter sites by emission tomography.
Various substituted pyrrolo isoquinolines have been described, including those with halo-substituents without, however, providing compounds that can be efficiently labeled with isotopic halogen to provide useful compounds for PET and/or SPECT imaging. Maryanoff, B. E. et al., U.S. Pat. No. 4,719,216 describes a variety of pyrrolo-isoquinolines, including some with a halogen substituent in the non-heterocyclic ring, a fluorine substituent on the heterocyclic ring and others with a perfluoro alkyl substituent on a phenyl group substituted at the 4-position. Synthesis required introduction of halogen at an early synthetic step. Related patents are U.S. Pat. Nos. 4,595,688 and 4,837,328. The latter describes a method of stereoselective synthesis at the 10b carbon. Maryanoff, B. E. et al., U.S. Pat. No. 4,908,450, describes certain salts of the same compounds and an alternative stereoselective reduction step during synthesis, European Patent 0 130 069 B1 is also related to the above U.S. patents. No utility for PET or SPECT imaging was disclosed.
Maryanoff, B. E., U.S. Pat. No. 4,713,386 has also described tetrahydroazeto [2,1-A] isoquinolines as compounds useful for treating depression. Halogen substituents can be present on the non-heterocyclic ring and also on a phenyl group substituted at the 4-position. No synthetic route allowing rapid substitution of an isotopic halogen as a late step of synthesis was described. No utility for PET or SPECT imaging was described.
Carson, J. R. et al., U.S. Pat. No. 4,572,911 described hexahydroindolinzine compounds having antidepressant properties. The compounds included those having a phenyl group at the 4 position which could have a halogen substituent. Halogen introduction was accomplished at an early step of the synthesis. No utility for PET or SPECT imaging was disclosed.